Inspection of tubes or other conduits for defects or wear within the walls is a necessary and time-consuming task for owners and operators of processes and equipment wherein high pressure fluid is transferred. Although many techniques have been developed and used in the prior art for inspecting newly manufactured tubing, it is the inspection of installed tubing which presents the greatest challenge to those in the industry. This is due to two factors: the difficulty in accessing sections of the installed tubing which may be within a tube bank or pressure vessel, as well as the possibility that significant structural degradation is likely to have occurred after an extended period of operation under conditions of high pressure, high temperature, and/or corrosive environment.
One such application wherein inspection of the structural integrity of an installed tube is critical is in the steam generators of a nuclear powered electric generating plant. In such a plant, the heated primary coolant flows through a plurality of inverted U-shaped tubes immersed in water from which steam is generated.
As it is typical in such an application for the primary side coolant pressure to be higher than that of the secondary, steam producing side, it is apparent that a failure of a steam generator tube will cause a leakage from the primary coolant into the secondary system.
The occurrence of such leakage is not unusual in such units, especially after extended periods of operation. For this reason, inspection of the individual steam generator U tubes is performed at regular intervals while the plant is shut down for service. An effective test program will not only locate specific failures that have already occurred in the steam generator tubing, but also attempt to identify the specific locations where a failure may be likely to soon occur. Thus, a high accuracy and sensitivity in the testing apparatus is a very desirable feature and has been the goal of the test apparatus heretofore in use and well known in the prior art.
One method of sensing anomalies in a metal structure utilizes the electrical eddy currents set up when a magnetic field and a conductive metal structure are moved relative to each other. The electrical eddy currents induced by such movements may be monitored by a magnetic field sensor and the results analyzed to determine the condition of the inspected metal structure. Eddy current probes are shown in U.S. Pat. No. 4,438,399 to Schnabl et al and U.S. patent application Ser. No. 693,427, filed Jan. 22, 1985 and assigned to the same assignee as the instant invention.
Another method of sensing anomalies in a metal structure utilizes ultrasonic wave patterns set up in the structure by one transducer and sensed by another.
In either inspection method, it would be useful to have a probe of simple and reliable design for both circumferentially and longitudinally traversing the interior of a nuclear steam generator tube, or the like, to be inspected. A simple, low cost rotating head for mounting the inspection sensors, whether they be eddy current coils or ultrasonic transducers, accordingly is provided. With the sleeving of failed tubes currently presenting an economically and technically beneficial alternative to the former practice of simply plugging off the failed tube in a nuclear steam generator, the need for such an inspection apparatus is readily apparent to those skilled in the art.